The present invention relates to an improved polishing apparatus. More specifically, the present invention relates to an improved apparatus for providing chemical-mechanical-planarization (CMP) to relatively large surfaces.
Various polishing pads for providing CMP to semiconductor surfaces are known and are described, for example, in U.S. Pat. Nos. 4,841,680; 4,927,432; and 4,728,552. Various slurries for use in providing CMP are also known and are described, for example, in U.S. Pat. Nos. 4,959,113; 5,264,010; 5,382,272; 389,352; and 5,391,258.
FIG. 1A shows a top view of a prior art polishing apparatus 100. FIG. 1B shows a side view of polishing apparatus 100 prior to initiating a polishing operation and FIG. 1C shows a side view of polishing apparatus 100 during the polishing operation. As will be discussed in greater detail below, apparatus 100 may be used to polish a surface 102 of a substrate 104.
Apparatus 100 includes a polishing pad assembly 110 and a support or chuck 120 mounted over assembly 110. Pad assembly 110 includes a lower pad 112 and an upper pad 114. Upper pad 114 provides an upper abrasive surface 116. The pads 112, 114 normally are configured so that upper pad 114 may be easily replaced when its abrasive surface 116 becomes worn. For convenience of illustration, upper pad 114 is shown separated from lower pad 112, however, the pads 112, 114 are normally in contact and are fixed relative to one another so that the rotation of upper pad 114 can be controlled by controlling the rotation of the lower pad 112. By way of example, polishing pads suitable for implementing pads 112, 114, are commercially available from Rodel of Newark, Del.
Support 120 is configured so that it may securely hold or clamp substrate 104 so that the substrate 104 remains substantially stationary with respect to support 120. Support 120 may be positioned as shown in FIG. 1B so that the surface to be polished 102 is separated from polishing pad assembly 110, and may also be positioned as shown in FIG. 1C so that the surface to be polished 102 is in contact with abrasive surface 116 of upper pad 114.
During a polishing operation, support 120 is movable so that the surface to be polished 102 may be moved into contact with abrasive surface 116 of upper pad 114 (as shown in FIG. 1C). Once the substrate is in contact with pad assembly 110, the pad assembly is rotated in the direction indicated by arrow 122 (shown in FIG. 1A) about axis of rotation 150. Further, support 120 is rotated in the direction indicated by arrow 124 about axis of rotation 152. Axes 150, 152 are both perpendicular to the plane of the page in FIG. 1A.
The rotation of pad assembly 110 and support 120 normally are controlled, for example, by one or more motors (not shown). Pad assembly 110 and support 120 rotate with respect to one another, however, they are normally not translated with respect to one another. Thus, axes 150, 152 remain substantially parallel and stationary with respect to one another. Since support 120 holds substrate 104 substantially stationary with respect to support 120, the rotation of pad assembly 110 and support 120 grinds the surface 102 of substrate 104 against the abrasive surface 116 of upper pad 114. The grinding of surface 102 against abrasive surface 116 polishes surface 102.
Abrasive surface 116 mechanically polishes surface 102. To improve the quality of polishing provided to surface 102, apparatus 100 is normally used in conjunction with a polishing slurry 130 (shown in FIG. 1C). Slurry 130 is normally poured onto the center of upper pad 114. As pad assembly 110 is roated, polishing slurry is distributed by centrifugal force and forms a relatively thin film on the entire abrasive surface 116 of upper pad 114. Slurry 130 includes a chemical polishing liquid (e.g., potasium hydroxide or ammonium hydroxide) and an abrasive (e.g., collodial silica or aluminum oxide) that is suspended in the liquid. The abrasive in slurry 130 cooperates with the abrasive surface 116 of upper pad 114 to mechanically polish surface 102. The chemical polishing liquid in which the abrasive is suspended is selected so that it chemically reacts with surface 102, thereby chemically polishing surface 102. Since apparatus 100 provides both mechanical and chemical polishing, the polishing process is referred to as chemical-mechanical-planarization (CMP).
One problem associated with polishing apparatus 100 relates to the distribution of slurry 130. Ideally, the slurry 130 is provided to all portions of the surface to be polished 102. Such a distribution permits an even amount of polishing to be provided to all parts of surface 102. However, polishing apparatus 100, as well as other prior art polishing apparatuses, fail to achieve this objective.
The relative motion of substrate 104 and pad assembly 110 defines a leading edge 140 (shown in FIG. 1C) and a trailing edge 142 of substrate 104. This relative motion causes the slurry 130 to build up in a "wave-front" 132 proximal to the leading edge 140. Some of the slurry 130 in the wave-front 132 penetrates between surface 102 and upper pad 114. However, there is an uneven distribution of the slurry beneath the substrate because more of the slurry reaches the outer edges of surface 102 than the center of surface 102.
FIGS. 2A and 2B illustrate the distribution of slurry across the surface 102 to be polished. FIG. 2A illustrates the distribution of slurry caused by rotation of pad assembly 110 (in the direction of arrow 122 as shown in FIG. 1A) when substrate 104 (and support 120) remains stationary and does not rotate. Under these conditions, a dashed line 210 represents a boundary between the portions of surface 102 that are wetted by the slurry and the portion that is not. Accordingly, the wetted portion is at 212 and the non-wetted portion is at 214. Specifically, the slurry wets the region 212 between the leading edge 140 of surface 102 and dashed line 210, however, the slurry does not reach the non-wetted region 214 to the right of dashed line 210 to the trailing edge 142.
FIG. 2B illustrates the distribution of slurry achieved when substrate 104 is rotated (in the direction of arrow 124 as shown in FIG. 1A) in addition to the rotation of pad assembly 110. In FIG. 2B, a dashed circle 220 represents the boundary between a wetted portion 222 and a non-wetted portion 224 of surface 102. As shown by FIG. 2B, rotation of surface 102 improves the distribution of slurry, however, a central region 224 remains essentially non-wetted with little or no slurry. So while the outer edge region 222 receives chemical and mechanical polishing, the central region 224 essentially receives only the mechanical polishing, or dry polishing, provided by the abrasive surface 116 of upper pad 114. Chemical polishing is normally a faster process than mechanical polishing. Apparatus 100 therefore tends to polish, due to the uneven distribution of slurry, the outer edge region 222 faster than the central region 224. The type of polishing provided by apparatus 100 is often referred to as being "edge-fast". Rather than being perfectly planar, after polishing by apparatus 100, surface 102 tends to be somewhat concave with region 224 bulging outward slightly rather than be planar with region 222.
Those skilled in the art will appreciate that the location and size of the central, non-wetted, region 224 are determined by several factors including, for example, the pressure between surface 102 and abrasive surface 116, the type of abrasive used in surface 116, the type of slurry used, the relative speeds of surface 102 and pad assembly 110, and, perhaps most importantly, the size of the surface to be polished 102. The tendency for the slurry to fail to wet the central region 224 increases with increases in the size of the surface to be polished.
Another factor that tends to make prior art polishing apparatus 100 provide an "edge fast" type of polish relates to the relative speeds of the central and outer portions of the surface to be polished 102. Referring to FIG. 1A, when support 120 is rotating about axis 152, the linear velocity of support 120 at the axis of rotation 152 is zero and this linear velocity increases as the distance from the axis of rotation 152 increases. As such, the outer portions of surface 102 move faster relative to the abrasive surface 116 than does the center of surface 102. This disparity in velocities also tends to provide a faster polishing to the edges of surface 102, thereby compounding the problems associated with the uneven distribution of slurry discussed above.
Both of the problems discussed above that contribute to making prior art polishing methods be "edge fast" become exacerbated by increases in the size of the surface to be polished 102. Therefore, although conventional polishing apparatus and techniques are satisfactory for providing CMP to semiconductor surfaces approximately eight inches in diameter these apparatuses and techniques have proven unsatisfactory for polishing or planarizing larger surfaces greater than eight inches. This is particularly true for polishing semiconductor surfaces larger than about fourteen inches in diameter.
One prior art method for improving the distribution of slurry 130 between surface 102 of substrate 104 and abrasive surface 116 is to provide micro-channels or perforations in the upper pad 114. However, such channels or perforations can cause breakage or scratching of the surface 102 to be polished.
Advances in the semiconductor industry continually lead to increases in the size of the wafers and chips being produced. There is therefore a need for apparatuses and methods for polishing relatively large semiconductor surfaces and other surfaces, particularly wafers sizes as large as fourteen inches in diameter.
It is therefore an object of the present invention to provide an improved polishing apparatus and method for polishing relatively large surfaces.
These and other objects of the present invention will be described in detail in the remainder of the specification referring to the drawings.